Michael Stockenhuber

Unusual structure and stability of iron-oxygen nano- clusters in Fe-ZSM-5 catalysts

Feng and Hall have recently reported that over-exchanged Fe-ZSM-5 materials prepared by a special route show high stability in the selective reduction of 2-methylpropane (isobutane) by NOx. We report that iron-oxygen clusters in ZSM-5 catalysts prepared by a conventional route show unexpected structural features, with significant iron-iron co-ordination and a rather short Fe- Fe interatomic distance of 2.53±0.02 Å. Iron in these clusters is readily interconvertible between the (II) and (III) oxidation states, but the clusters are stable against reduction to Fe(0) even at 1250 K in hydrogen. We suggest that the clusters adopt structures similar to that of ferredoxin II, Fe3S4.

Ambient temperature carbon monoxide oxidation using copper manganese oxide catalysts: Effect of residual Na+ acting as catalyst poison

The effect of ageing of copper manganese oxide catalysts prepared by co-precipitation is described and discussed. In particular, the ageing leads to a significant decrease in the concentration of surface sodium and this correlates with the catalytic activity for the oxidation of carbon monoxide at the extended ageing time.

Transition metal containing mesoporous silicas - redox properties, structure and catalytic activity

Abstract Mesoporous silicas of the MCM-41 type, containing iron, were prepared by synthesis and after framework synthesis, including deposition of iron from dried methanolic solution of iron(III) nitrate. Concentration of iron was up to 12 wt.%. In situ EXAFS, NO adsorption followed by IR spectroscopy and temperature programmed reduction suggest that the resulting iron in most cases is present mainly in the form of isolated iron entities. Depending on the preparation procedures, subtle differences in bonding of iron to the framework were observed. In the oxidation of large hydrocarbons to oxygenates with N 2 O the most active catalysts are aluminium containing materials. The main difference between FeZSM-5 and FeMCM-41 materials is the presence of Fe–O nanoclusters in FeZSM-5 which seem to be more active in the oxidation reaction than isolated iron entities in mesoporous materials.

Sorption of light alkanes on H-ZSM5 and H-mordenite

1. Abstract The sorption of light n- and iso-alkanes on H-ZSM5 and H-MOR was studied by means of calorimetry, gravimetry and in situ infrared spectroscopy. Alkanes sorb preferentially localized on the strong Bronsted acid sites of H-ZSM5 and H-MOR. Two n-alkane, but only one iso-alkane molecule can be simultaneously accommodated on the Bronsted acid sites of H-ZSM5. In contrast, only one alkane molecule interacts with a strong Bronsted acid site of H-MOR, and, moreover, only a third of these sites are strongly interacting at all with alkanes because of steric reasons. For both zeolites, the heat of adsorption increases with increasing size of the hydrocarbon. The linear relationship between the enthalpies and the entropies of sorption suggests that the molecular motions of the sorbed molecules are restricted in proportion to the strength of interaction with the zeolite.

Characterization and removal of extra lattice species in faujasites

The acidic properties of dealuminated Y-type zeolites were characterized by infrared (IR) spectroscopy, microcalorimetry, 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and temperature-programmed desorption (TPD). Microcalorimetric measurements exhibited a uniform heat of adsorption (140 kJ/mol) of ammonia on the strong Bronsted acid sites. The differences in the acid site concentrations measured by adsorption of ammonia from the gas phase and by decomposition of ammonium-exchanged zeolites are discussed. The results indicate that parts of the extra lattice material consisting of cationic aluminium oxide species or silica alumina species are removed by ion exchange with aqueous solutions of ammonium hydroxide. Based on this, a method for the controlled removal of extra lattice material was developed.

Selective catalytic reduction of NOx over microporous CuAPO-5: structural characterisation by XAS and XRD

The effects of copper source, copper content, Al : P ratio and silicon content for the hydrothermal synthesis of CuAPO-5 using tetraethylammonium hydroxide as template have been investigated. The copper to phosphorus ratio in the synthesis gel was varied from 0.04 to 0.2, and copper(II) oxide and copper(II) acetate were used as copper sources. The samples were characterised by XAS (XANES, EXAFS), XRD (X-ray diffraction), TGA (thermogravimetric analysis), SEM (scanning electron microscopy) and elemental analysis. Rietveld refinements of the structure of CuAPO-5 were carried out on synchrotron X-ray powder diffraction data. The catalytic properties of CuAPO-5 in selectively reducing NOx in the presence of hydrocarbons (SCR-HC) were studied. The results from EXAFS show that copper is present in a distorted octahedral environment, with 4 or 5 Cu–O distances at 1.95 A and 1 or 2 Cu–Al/P distances at about 3.14 A for the as-synthesised samples. The calcined samples still show the same four Cu–O distances at 1.95 A, but with the composite peak at 3.14 A being significantly reduced. The turquoise colour of the as-synthesised CuAPO-5 is consistent with the copper atoms being complexed by water molecules within the pores as the hexaaquacopper(II) complex. The extraframework complex cation is electrostatically bound to the anionic phosphate sites of the framework. The colour changes to olive green on calcination, indicating tetrahedrally coordinated framework copper. The selective catalytic reduction of NOx by propene is catalysed by CuAPO-5 to the extent of 18% conversion. The Rietveld analysis show that calcined CuAPO-5 belongs to the P6/mcc spacegroup in which Al and P are indistinguishable.

The local structure of aluminium sites in zeolites

The long range ordering and thus the average structure of crystalline zeolites can be determined by various diffraction and spectroscopic techniques. It has, however, proved difficult to establish the local structure surrounding these aluminium sites by diffraction methods. The most useful information has come from theoretical studies (M. Brandle et al., J. Chem. Phys., 1998, 109, 10379; U. Eichler et al., J. Phys. Chem. B, 1997, 101, 10035) which suggest that the Al–O distance associated with the proton is longer than other Al–O interatomic distances. Employing in situ X-ray absorption fine structure spectroscopy (EXAFS) of the aluminium edge at 1565.6 eV, we report individual bond lengths angles for the local aluminium environment of neutral and acidic zeolites. For two acidic zeolites we find that there is indeed one Al–O distance that is significantly longer than those in a neutral material. We also show that for the average T-atom positions our EXAFS results are consistent with X-ray diffraction measurements, to an accuracy of ca. 0.01 A. Changes in bond angles show how the zeolite structure distorts to accommodate Bronsted acidity.

Modelling aromatics in siliceous zeolites: a new forcefield from thermochemical studies

A new forcefield for the modelling of interactions between aromatics and siliceous zeolites has been derived by fitting to calorimetric data on the sorption of benzene in siliceous faujasite. The calometric measurements suggest a heat of sorption of 55 kJ mol−1 at densities lower than 22 molecules per unit cell. Monte Carlo docking calculations have been carried out, using the new forcefield, to predict the most favourable binding sites for sorption of benzene in this zeolite. They show a relatively flat potential-energy surface as compared to Na-Y with a wide variety of different mainly low-symmetry binding sites. The sites are to be classified as being either near to 4-rings (58.4 kJ mol−1), 6-rings (50.0 kJ mol−1) or in the 12-ring window (43.7 kJ mol−1). These results are consistent with neutron diffraction measurements on the same system. Molecular dynamics calculations with the new forcefield suggest that two processes contribute to the motion of benzene molecules in the pore system. At low temperatures, the benzene molecules tend to be confined to a single supercage by sliding around the walls. However, at higher temperatures, the molecules have sufficient kinetic energy to move through the 12 ring window into an adjacent supercage. Minimum-energy pathways have also been calculated, based on the docking binding sites, which correlate well with this rationalisation of the transport mechanism.

Formation of PCDD/Fs in Oxidation of 2-Chlorophenol on Neat Silica Surface.

This contribution studies partial oxidation of 2-chlorophenol on surfaces of neat silica at temperatures of 250, 350, and 400 °C; i.e., temperatures that frequently lead to catalytic formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) from their precursors. We have identified 2,6-dichlorophenol (2,6-DCPh), 2,4-dichlorophenol (2,4-DCPh), and 2,4,6-trichlorophenol (2,4,6-TriCPh), but have detected no chlorinated benzenes (CBzs). The detected chlorinated and nonchlorinated DD/Fs comprise dibenzo-p-dioxin (DD), 1- and 2-monochlorodibenzo-p-dioxin (1-, 2-MCDD), 1,6-, 1,9-, 1,3-dichlorodibenzo-p-dioxin (1,6-, 1,9-, 1,3-DCDD), 4-monochlorodibenzofuran (4-MCDF), and 4,6-dichlorodibenzofuran (4,6-DCDF) at the reaction temperatures of 350 and 400 °C. However, at a lower reaction temperature, 250 °C, we have detected no PCDD/Fs. We have demonstrated that neat silica surfaces catalyze the generation of PCDD/Fs from chlorophenols at the upper range of the catalytic formation temperature of PCDD/F. The present finding proves the generation of PCDD/Fs on particles of fly ash, even in the absence of transition metals.

Catalytic combustion of ventilation air methane (VAM) – long term catalyst stability in the presence of water vapour and mine dust

In this paper, we report new insights into the deactivation phenomenon of palladium based catalysts for catalytic combustion of ventilation air methane (VAM). It was found that the primary factor responsible for low temperature catalyst deactivation is the water vapour present in the feed stream. The influence of water vapour on VAM was examined by comparing the properties of fresh catalysts with catalysts following over 1000 h reaction time-on-stream. The techniques applied to characterize the catalysts included TPD, XRD, N2-isotherm adsorption, H2-chemisorption and XPS analyses. Alternating between dry and water vapour-saturated VAM feed disclosed ca. 50% reversible drop in activity. XPS analysis suggests an absence of a palladium hydroxide phase during the initial 2 h on stream, although prolonged exposure to the reactant leads to the formation of palladium hydroxide, which appears to match the progressive deactivation of the Pd/Al2O3 catalyst. Introduction of VAM dust (a mixture of fine coal, CaCO3 and aluminosilicate particles) causes a variation in catalytic activity originating from coal-dust ignition and the effect of chloride on the surface of the catalyst. In the presence of these inhibiting agents, an average methane conversion of higher than 75% over 1100 h was achieved at reaction temperatures below 600 °C.